1) Field of the Invention
The present invention relates to an optical modulator suitable, for example, for use in the field of optical communications.
2) Description of the Related Art
An optical modulator using an electro-optical crystal such as an LiNbO3 (lithium niobate, which will hereinafter be referred to simply as LN) substrate or an LiTaO2 substrate is formed by placing electrodes in the vicinity of an optical waveguide, which is formed in a manner such that a metal film is formed at a portion of the crystal substrate and thermally diffused or proton-replaced in benzoic acid after patterning to form an optical waveguide.
Usually, as the optical waveguide of the optical modulator, there is employed a Mach-Zehnder optical waveguide comprising an incidence waveguide, two parallel waveguides and an outgoing waveguide, with a signal electrode and an earth electrode being provided on the parallel wageguide to form a coplanar electrode. In the case of the employment of a Z-cut substrate, an electrode is located just above the optical waveguide because of the utilization of a variation of refractive index due to an electric field in the Z direction. At this time, a signal electrode and an earth electrode are patterned on parallel waveguides, respectively, and for the purpose of preventing the light propagated in the parallel electrodes from being absorbed by the signal electrode and the earth electrode, a buffer layer is interposed between the LN substrate and the signal electrode/earth electrode. As the buffer layer, for example, there is employed SiO2 having a thickness of approximately 0.2 μm to 1 μm.
For driving the optical modulator at a high speed, the signal electrode and an endpoint of the earth electrode are connected to each other through a resistor to establish a traveling-wave electrode and a microwave signal is applied thereto from the input side. At this time, due to the electric field, the refractive indexes of the two parallel waveguides vary like +Δna and −Δnb and the phase difference between the two parallel waveguides varies so that a signal light intensity-modulated is outputted from the outgoing waveguide.
However, the absolute values of the electric fields applied to the two parallel waveguides differ from each other, and since Δna<Δnb, a phenomenon (chirp) occurs where the wavelength of the output light varies in the transition phase from the ON-state of the light to the OFF-state thereof. For solving this, for example, there has been proposed the employment of a substrate in which a polarization inversion is made as shown in FIG. 9. In an optical modulator 100 shown in FIG. 9, a substrate is designated at numeral 106 and is equipped with an area 106-1 and a polarization inverting area 106-2 in which the electro-optical characteristic, i.e., the phase modulation characteristic of the light propagated in an optical waveguide in conjunction with an applied voltage, is in an inverted condition with respect to the area 106-1.
In addition, in FIG. 9, an optical waveguide 102 is formed on the substrate 106 and is composed of an incidence side Y-branched waveguide 103, two parallel waveguides 104-1, 104-2 and an outgoing side Y-branched waveguide 105, and a signal electrode 101a passes on one parallel waveguide 104-1 in the non-inverting area 106-1 and passes on the other parallel waveguide 104-2 in the polarization inverting area 106-2. Moreover, numeral 101b designates an earth electrode, and 108 depicts a buffer layer interposed between the substrate 106 and the electrodes 101a, 101b. 
In the optical modulator 100 shown in FIG. 9, in a case in which the overall length L1 of the non-inverting area 106-1 in an optical propagation direction is equal to the length L2 of the inverting area 106-2 in an optical propagation direction, the lights passing through the two parallel waveguides 104-1 and 104-2 respectively vary by +Δθs and −Δθg in the non-inverting area 106-1 and espectively vary +Δθg and −Δθs in the polarization inverting area 106-2.
Accordingly, the phases of the lights passing through the two parallel waveguides 104-1 and 104-2 respectively vary by +(Δθs+Δθg) and −(Δθs+Δθg) in the Y-branched waveguide 105, which provides a phase modulation so that the absolute values are equal to each other but the signs being reverse to each other. This enables the wavelength chirp to become zero with respect to the light to be outputted from the Y-branched waveguide 105. Moreover, the adjustment of the chirp quantity becomes feasible by changing the ratio between the aforesaid L1 and L2.
FIG. 10A is a cross-sectional view taken along a line A-A′ in the optical modulator 100 shown in FIG. 9, FIG. 10B is a cross-sectional view taken along a line B-B′ in the optical modulator 100 shown in FIG. 9, and FIG. 10C is a cross-sectional view taken along a line C-C′ at the boundary portion between the non-inverting area 106-1 and the polarization inverting area 106-2 in the optical modulator 100 shown in FIG. 9. The signal electrode 101a is formed on the parallel waveguide 104-2 in the non-inverting area 106-1 and on the parallel waveguide 104-1 in the polarization inverting area 106-2. Therefore, as shown in FIG. 10C, at the boundary portion between the non-inverting area 106-1 and the polarization inverting area 106-2, the signal electrode 101a makes a connection between the upper portions of the parallel waveguide 104-1 and the parallel waveguide 104-2.
In addition, in the case of such an optical modulator using a substrate having electro-optical effects, an optical waveguide having a ridge configuration (structure) is obtainable in a manner such that both external sides of an optical waveguide (in FIG. 9, the parallel waveguides 104-1 and 104-2) in an interactive area are dug through the use of etching or the like to form grooves. In comparison with an optical waveguide formed on a flat substrate without having grooves, an optical waveguide with this ridge configuration is capable of improving the electric field application efficiency to this optical waveguide when an electric field is applied through electrodes, lowering the drive voltage and enhancing the broadbandization in modulation-possible optical wavelength. Still additionally, because of a high containment of light to the waveguide, in particular, also in the case of the formation of a waveguide having a curve with a small radius, the loss is reducible.
Therefore, through the use of an optical modulator having the aforesaid polarization inverting area and having a ridge configuration, it is expectable to achieve higher light containment in a waveguide, reduction of drive voltage and the enhancement of broadbandization while advancing the aforesaid wavelength chirp suppression. In this case, for example, it is considered that an optical modulator 100A is constructed as shown in FIG. 11 or FIGS. 12A to 12C. FIG. 12A is a cross-sectional view taken along a line A-A′ in the optical modulator 100A shown in FIG. 11, FIG. 12B is a cross-sectional view taken along a line B-B′ in the optical modulator 100A shown in FIG. 11, and FIG. 12C is a cross-sectional view taken along a line C-C′ at the boundary portion between a non-inverting area 106-1 and a polarization inverting area 106-2 in the optical modulator 100A shown in FIG. 11.
In the optical modulator 100A shown in FIGS. 11 and 12A to 12C, grooves 107 are made at both external sides of parallel waveguides 104-1 and 104-2, thereby enabling the parallel waveguides 104-1 and 104-2 to provide an optical waveguide with the aforesaid ridge configuration. Additionally, a ridge configuration is formed outside the parallel waveguides 104-1 and 104-2 having the ridge configuration.
Furthermore, for the construction of the aforesaid optical modulator 100A shown in FIG. 11, in addition to the optical waveguide 102, an signal electrode 101a and an earth electrode 101b are formed on a substrate 106 in which the grooves 107 are formed at both the external sides of the parallel waveguides 104-1 and 104-2. Also in this case, the signal electrode 101a makes a connection between the upper portions of the parallel waveguide 104-1 and the parallel waveguide 104-2 in the boundary portion between a non-inverting area 106-1 and a polarization inverting area 106-2.
Incidentally, the well-known techniques connected with the present invention, there are the following patent documents 1 to 4.
[Patent Document 1]                Japanese Patent Laid-Open No. 2003-228033        
[Patent Document 2]                Japanese Patent Laid-Open No. HEI 11-101962        
[Patent Document 3]                Japanese Patent Laid-Open No. 2001-194637        
[Patent Document 4]                Japanese Patent Laid-Open No. 2003-233047        
However, in the case of realizing both the aforesaid polarization inverting area and ridge waveguide in a single optical modulator, a signal electrode comes down to the bottom of a groove at the boundary portion between the polarization inverting area and the non-inverting area and, hence, a disconnection of the electrode tends to occur at this portion. In addition, when the electrode comes into contact with a side surface of the ridge, a discontinuity of the characteristic impedance (impedance mismatch) occurs, which interferes with the enhancement of broadbandization of a modulator.
For example, for the construction of the aforesaid optical modulator shown in FIGS. 11 and 12A to 12C, in the construction, the signal electrode 101a falls to the bottom of the groove 107 at the portion D (see FIGS. 11 and 12C) forming the boundary between the non-inverting area 106-1 and the polarization inverting area 106-2 and, hence, a disconnection of the signal electrode 101a tends to occur at this portion, and when the electrode 101a comes into contact with a side wall of the ridge structure of the parallel waveguides 104-1 and 104-2, a discontinuity of the characteristic impedance can occur.
Incidentally, each of the techniques disclosed in the aforesaid patent documents 1 to 4 cannot eliminate the above-mentioned problems arising in a case in which both the polarization inverting area and ridge waveguide are realized in a single optical modulator.